The band pass filter in a communication system is comprised of various acoustic wave devices, including surface acoustic wave filter and bulk acoustic wave filter. Acoustic wave devices are very sensitive to the mass loading. Even a very small mass loading may change the frequency response of the device. It is known that an 8 nm-thick titanium mass loading layer can lower the resonance frequency of a film bulk acoustic wave resonator (FBAR) by 10 MHz, and the return loss is reduced accordingly. This is owing to the mass loading on the surface of the device changes the boundary condition of the acoustic wave resonance, which leads to a change of the frequency response characteristic of the device. Therefore, it should be avoided to coat any material on the surface of the device during the fabrication and packaging processes. According to the results of product analysis, a cavity on the acoustic wave resonance or transmission area is requested for both surface and bulk acoustic wave devices to avoid the surface mass loading effect described above.
Currently, the surface acoustic wave filters used in mobile communication systems are mostly bonded to the surface of the packaging substrate by using the ultrasonic flip chip bonding technology. The space between the chip and the packaging substrate is about 10 μm, which can avoid molding compound flowing onto the surface of the surface acoustic wave device during the molding process. However, it is required to use the high price gold bump in the ultrasonic flip chip bonding technology, and the fabrication rate is low.
In order to reduce the deficiency of the fabrication process using the ultrasonic flip chip bonding technology, some of the product are bonded by using copper pillar reflow flip chip bonding technology. However, the space between the chip and the packaging substrate is determined by the height of the copper pillars, which is usually higher than 20 μm. In this case, the molding compound can flow onto the surface of the surface acoustic wave device more easily during the packaging process and change the frequency response of the device.